1. Field of the Invention
The present invention relates to the manufacture of high brightness (white) low abrasion calcined clay pigments from crude (unrefined) hydrated kaolin clay ores. Generally, such processes include an initial "wet processing" of the hydrated clay to refine the clay, followed by removal of substantially all of the water introduced during wet processing to provide dry pulverulent fine particle-size hydrated kaolin which is then heat treated (calcined) at elevated temperature to remove water originally chemically associated with the clay particles and thereby impart a spectrum of desirable functional properties not possessed by refined hydrated kaolin clay pigments. In particular, the invention is directed to reducing the energy required to produce such pigments.
High brightness low abrasion calcined kaolin clay are known in the art. The product commercially available as ANSILEX.RTM. clay is an example. This pigment is now widely used as a paper filling pigment and it also finds application in paper coating use.
Methods for preparing calcined kaolin clay pigments are described in the following: U.S. Pat. No. 3,941,872: U.S. Pat. No. 3,798,044; U.S. Pat. No. 3,586,523; U.S. Pat. No. 3,519,453; U.S. Pat. No. 3,383,438; U.S. Pat. No. 3,171,718; U.S. Pat. No. 3,309,214; U.S. Pat. No. 3,021,195; U.S. Pat. No. 3,058,671; U.S. Pat. No. 3,014,836; British Pat. No. 1,181,491. The processing described in these patents encompasses variations in the feed material to the calciner and calcination conditions to produce calcined clay pigments of desired brightness and low abrasivity. For example, a fine particle-size fraction of a hard kaolin clay crude (ememplified by certain deposits of gray Gerogia Kaolin) is employed in practice of the process described in U.S. Pat. No. 3,586,523; fine particle-size fraction mechanically delaminated kaolin obtained by delaminating chunks of large booklets of kaolinite and fractionating to remove the fine particle-size fraction is used in some of the other processes for providing low abrasion pigments.
In general, processes for producing low abrasion white calcined clay pigments are characterized by the fact that substantial amounts of water must be added to the dry crude clay to permit the crude clay to be refined ("wet-processed") before the clay is charged to a calciner. Wet processing includes, but is not limited to, placing the crude clay into the form of a fluid aqueous suspension, removing coarse grit and then recovering a desired fine particle-size fraction of hydrated clay. The fine particle size is a requirement inter alia for low abrasiveness of the ultimate calcined clay product. Fractionation is conventionally carried out by centrifugation of a chemically deflocculated (dispersed) aqueous suspension of the degritted clay. For fractionation to be effective to recover a fine particle-size fraction (generally, 90% finer than 2 microns e.s.d. or finer), the feed to centrifuge must be well dispersed and sufficiently dilute; for example, below 50% clay solids. The fine size fraction is obtained in the form of a dispersed suspension as a centrifuge overflow and is at somewhat lower solids than the feed to the centrifuge. For example, the fine particle size fractions may be recovered at 35 to 40% solids from a centrifuge charged with feed slurry at 45-50% solids.
All liquid water introduced during wet processing of kaolin clay used in the manufacture of calcined clay pigments must be removed from refined kaolin clay prior to calcination. The reason is that the calciner must be charged with dry pulverized clay in order to manufacture a pigment having reasonably low abrasivity; for example, a Valley Abrasion Index below 200, preferably below 100, and most preferably appreciably below 50. The water contained in a slurry of wet processed kaolin presents a formidable filtration and/or evaporative load in a large scale commercial operation. For example, a plant operating to produce 40 tons per day of dry pulverized calciner feed from wet processed feed slurry at 40% solids, would be faced with the problem of removing 60 tons of water per day. This is by no means a simple or inexpensive task. When the clay is composed of extremely fine particles, for example, 90% finer than 1 micron, dispersed aqueous suspensions of the kaolin will form dense impermeable cakes resistant to further efficient removal of water when processed in conventional dewatering equipment such as cyclones, rotary vacuum filters, or plate-and-frame filters. To facilitate dewatering, an acidic flocculating agent must be added. However, even when the suspensions are flocculated, the conventional solids-liquids separation equipment removes only limited amounts of water from suspensions of fine particle-size fractions of kaolin clay such as required as the source of low abrasion calcined clay pigments. Furthermore, filtration rates are low compared with rates achieved using coarser clay particles. However, use of the large clay particles would not be suitable for manufacture of low abrasion calcined kaolin pigments.
Water remaining in the solid filter cakes obtained by rotary vacuum or plate-and-frame aqueous filtration of suspensions of fine particle-size kaolins can be removed by thermal means. This is conventionally done by adding a dispersant (deflocculating agent) to transform the cake into a fluid slurry from which water is removed by spray drying. However, such practice results in the introduction of undesirable salts unless expensive washing is practiced.
Conventional dewatering equipment such as mentioned above consumes considerable energy when utilized in large-scale commerical plants. Considerable electrical energy is called for in the operation of rotary filters, cyclones, and the pumps. Fossil fuel is needed for spray drying. The electrical energy requirement could be reduced by omitting the filtration step and directly spray drying the dispersed slurry of wet processed clay to remove essentially all water. This would also avoid excessive salts in the dried clay; however, fuel energy requirements would be increased significantly.
With the escalating cost of fuel and concern for its availability, there is an obvious need for reducing the energy expended in manufacture of calcined clay pigments, especially so because the calcination operation that follows dewatering must be carried out at elevated temperatures, generally above 1500.degree. F., utilizing equipment that is usually limited in energy efficiency because of the need to avoid local overheating which will result in undesirable residue and/or abrasiveness in the finished product. Given the necessity for providing high brightness calcined clay pigments having low abrasion values and limitations in processing options resulting from these requirements, conventional clay dewatering technology has not been adequate to permit striking improvements in the energy efficiency of processing schemes.
2. Prior Art
There is a long history of attempts to dewater slips of clays, inclusive of kaolins, by filters of various designs utilizing electrokinetic phenomena such as electrophoresis and electroosmosis. Reference is made to U.S. Pat. No. 1,229,203 to Schwerin (1917) which is directed to electrically dewatering dispersed suspensions of fine particle-size fractions of kaolin clay. Recently, interest in electrofilters has revived in response to the desire of certain consumers of hydrated (uncalcined) kaolin clay pigments to purchase shipments of clay pigments in the form of dispersed fluid high solids (e.g., 70%) slurries instead of dry powders while avoiding certain expenses entailed in the conventional technique for preparing such concentrated slurries. Electrofiltration was envisioned simply as a means for avoiding the expense incurred in removing water from a substantial amount of clay by spray drying and subsequently reintroducing water into previously dried clay by mixing it with a filter cake obtained, for example, by conventional rotary vacuum filtration. Reference is made to the following patents: U.S. Pat. No. 4,003,811; U.S. Pat. No. 4,003,819; U.S. Pat. No. 4,048,038; and French Demande Pat. No. 2,354,802. In other words, electrofiltration is now comtemplated simply as a technique for transforming a 60% solids flocculated (acidic) filter cake into a 70% solids deflocculated fluid slurry of hydrated kaolin clay suitable for shipment to a consumer.
It has been commercial practice to utilize the heat in calciner stack gases to preheat clay suspensions before drying them in spray dryers.
To the best of my knowledge, the general concept of incorporating an electrofilter as part of an improved system for producing calcined clay is novel, as are the specific concepts of preheating feed to an electrofilter with hot calciner exhaust gases and utilizing the electrofilter to dewater relatively dilute, e.g., 40% solids, slurries of dispersed kaolin to levels suitable for spray drying, e.g. 55% to 60% solids.